Whether you're a nervous or curious flyer, you may wonder about some of the sounds you hear while on the plane. Remember, planes operate using multiple mechanical systems. As the components of these systems move, various noises will occur. Depending on where you're sitting on the plane, some of the sounds might be louder than others. These four noises are expected during regular operations and are not cause for concern.

CHIMING SOUNDS
During a flight, you may hear chimes or doorbell sounds. These have to do with communication between the crew and the passengers. A single chime sounds when a passenger has pressed their call button. A high-then-low chime means flight attendants are communicating via the intercom or that the flight deck is communicating with a flight attendant. 

Shortly after takeoff, you might hear two similar-sounding tones. These signal that the plane has reached 10,000 feet in altitude and let crew members know the aircraft is in critical stages of the flight—during which they must be attentive and refrain from nonessential conversation. After this chime sounds, you'll typically see the crew get up and prepare for in-flight beverage service. 

BARK-LIKE SOUNDS
If flying on the Airbus A320 or A330, chances are you will hear a sound resembling a barking dog at some point. This comes from a hydraulic pump system called the Power Transfer Unit (PTU). Whenever the system detects a drop in pressure between two of the plane's hydraulic systems, the PTU works to transfer the hydraulic power. This sound will be heard most loudly by those seated near the wings. 

BANGING SOUNDS
When you're on the plane but it hasn't left the gate, the frequent banging sounds are luggage and other items being loaded into the plane's cargo section. The loudest of such noises, often accompanied by a thudding noise, is the cargo door being closed. Another banging noise right before landing is typically the landing-gear doors being opened and the wheels being deployed. 

ENGINE NOISES
While the hum of the engines is going to be heard throughout the flight, the noise level may change. In various phases of flight, as more or less power is needed, you may experience changes in engine sounds. For example, less power is needed to descend (less noise) while more power is needed to turn (more noise).